4 ms·

Dumb question: why did we need to see far-away galaxys moving away faster than near galaxies to conclude that the universe was expanding? Wouldn’t just the fact

Dumb question: why did we need to see far-away galaxys moving away faster than near galaxies to conclude that the universe was expanding? Wouldn’t just the fact that everything is moving away from us lead to the same conclusion?

AStonesThrow 1y ago
That is not a fact; there are stars that are moving closer to us, and the Andromeda Galaxy is expected to "collide" with the Milky Way at some future point.
kryptiskt 1y ago
If we see farther away galaxies moving away at the same speed as nearby galaxies that means that all the expansion of the universe happens between us and the nearby galaxies (since the farther away galaxies wouldn't be moving away from the nearby galaxies). So we would have some kind of local expansion of space around our galaxy. This is if we see a redshift in all directions, if we see a redshift in one direction and a blueshift in the opposite direction, that just means that our galaxy is moving relative to the observed galaxies (also, such a dipole can be seen in the microwave background).
JumpCrisscross 1y ago
> why did we need to see far-away galaxys moving away faster than near galaxies to conclude that the universe was expanding? Wouldn’t just the fact that everything is moving away from us lead to the same conclusion? Imagine inflating a balloon onto which you've painted dots. All the dots move apart. But the ones furthest apart move apart faster than those close together. This is how you know the dots aren't just moving in their local environment, the entire space is expanding (everywhere). (If you want a 1D representation, move your fingers apart at a constant rate. Consider how much further apart your pinky and index finger are compared with your middle and ring finger. That wouldn't happen if you just make a Spock hand.)

4 ms·

Dumb question: why did we need to see far-away galaxys moving away faster than near galaxies to conclude that the universe was expanding? Wouldn’t just the fact

by Fraterkes 1y ago

AStonesThrow 1y ago
That is not a fact; there are stars that are moving closer to us, and the Andromeda Galaxy is expected to "collide" with the Milky Way at some future point.
kryptiskt 1y ago
If we see farther away galaxies moving away at the same speed as nearby galaxies that means that all the expansion of the universe happens between us and the nearby galaxies (since the farther away galaxies wouldn't be moving away from the nearby galaxies). So we would have some kind of local expansion of space around our galaxy. This is if we see a redshift in all directions, if we see a redshift in one direction and a blueshift in the opposite direction, that just means that our galaxy is moving relative to the observed galaxies (also, such a dipole can be seen in the microwave background).
JumpCrisscross 1y ago
> why did we need to see far-away galaxys moving away faster than near galaxies to conclude that the universe was expanding? Wouldn’t just the fact that everything is moving away from us lead to the same conclusion? Imagine inflating a balloon onto which you've painted dots. All the dots move apart. But the ones furthest apart move apart faster than those close together. This is how you know the dots aren't just moving in their local environment, the entire space is expanding (everywhere). (If you want a 1D representation, move your fingers apart at a constant rate. Consider how much further apart your pinky and index finger are compared with your middle and ring finger. That wouldn't happen if you just make a Spock hand.)